This invention relates generally to a color picture tube apparatus, and more particularly, to a color picture tube apparatus provided with a deflection system that corrects the aberration of vertical magnetic deflection by which plural electron beams are influenced, and an in-line type electron gun.
In general, a color picture tube is provided with a screen inside the panel of an evacuated envelope, to which phosphors of three separate sets are uniformly applied in a stripe shape or in a dot shape, and the respective phosphors emit a red, green or blue light, respectively.
Three electron guns are provided corresponding to such phosphors of three colors, and three electron beams discharged by the three electron guns are caused to impinge through a large number of apertures of a color-selection electrode, i.e., shadow mask, on the corresponding phosphors, which, in turn, are excited. During the passage of the electron beams, horizontal and vertical deflection magnetic fields deflect these electron beams so as to scan the screen.
However, the rasters drawn by scanning of these electron beams are not converged on the screen due to the following reasons.
(a) Because the respective electron beams are discharged from the electron guns which are disposed at separate different positions, each of beams pass through different positions in the deflection magnetic field. Thus the amounts of deflection that the respective electron beams undergo are different.
(b) The distance between the center of deflection and the screen does not coincide with the radius of curvature of the screen.
The most simplified configurations to cause the rasters to coincide with each other use a plurality of electron beams in an in-line arrangement, and the deflection magnetic fields are non-uniform. Specifically, a pin-cushion type horizontal deflection magnetic field and a barrel type vertical deflection magnetic field are used. With this design, the rasters of the side electron beams substantially can be converged. However, the rasters of the side electron beams do not converge on the rasters of the center electron beams. Specifically, the rasters of the center electron beams become smaller than the rasters of side electron beams. The difference in size between the center and side beam rasters is called a coma error, and in the case of 14-inch type color picture tube, both a vertical direction coma error (VCR) and a horizontal direction coma error (HCR) occur on the order of 1 to 2 mm. In order to correct this difference so as to automatically converge the rasters (self convergence), magnetic pieces have been disposed on the side of the deflection magnetic field to locally adjust the magnetic field. This configuration was disclosed in U.S. Pat. No. 3,860,850 issued to Takenaka et. al.
However, the requirements for high definition of the screen necessitate the increase of the horizontal deflection frequency, and an apparatus provided with a horizontal deflection frequency of as high as 64 kHz, four times the frequency of the conventional TV apparatus, has been practically used. In this case, the above-described configuration that employs the magnetic pieces cannot sufficiently adjust the magnetic field because of losses within the magnetic pieces caused by the increase of the deflection frequency. When such magnetic pieces are omitted, the horizontal direction coma error (HCR) can be reduced by improving the distribution of horizontal deflection coils, however, the correction of the vertical direction coma error (VCR) is more difficult.
As a result of this, a sub-coil for use in correction has been attached in place of the magnetic pieces on the main vertical deflection coil, (as described in Japanese Utility Model Publication No. 57-45748). In this case, a pair of sub-coils coiled around a U-shaped core are disposed between the pointed end of the electron gun of the picture tube and the front side of the main deflection coil in such a manner that they oppose each other in the vertical direction. The magnetic fields generated by these sub-coils are of pin-cushion type, and are superimposed on the vertical deflection magnetic field. With the sub-coils, the coma error of the vertical direction can be reduced to approximately 0.2 mm in the case of a 14-inch type color picture tube, but cannot be completely eliminated. This means that the rasters at the intermediate portion of the screen undergo locally excessive correction. Even small coma errors of such extent as described above can develop shear in color that causes color distortion with respect to the characters displayed on the screen in the case of a high-definition color picture tube for use in a computer display and the like.